Traveling-wave tube slow-wave structure including multiple helices interconnected byspaced conductive plates



Aug. 23, 1966 MANN 3,268,761

TRAVELING-WAVE TUBE SLOW-WAVE STRUCTURE INCLUDING MULTIPLE HELICES INTERCONNECTED BY SPACED CONDUCTIVE PLATES Filed April 5, 1963 5 Sheets-Sheet 1 @Li I HI- HIIIIIIIIIHI- AW,

M. M. MANN 3,268,761 WAVE TUBE SLOW-WAVE STRUCTURE INCLUDING MULTIPLE Aug. 23, 1966 TRAVELING HELICES INTERCONNECTED BY SPACED CONDUCTIVE PLATES Filed April 5, 1963 5 Sheets-Sheet 2 man/2%. Mama W. 1444 44 ay 3,268,761 TRAVELING-WAVE TUBE SLOW-WAVE STRUCTURE INCLUDING MULTIPLE Aug. 23, 1966 M. M. MANN v HELICES INTERCONNECTED BY SPACED GONDUGTIVE PLATES Filed April 5, 1963 3 Sheets-Sheet 3 Ava/05% United States Patent 3,268,761 TRAVELING-WAVE TUBE SLOW-WAVE STRUC- TURE INCLUDING MIULTIPLE HELICES INTER- CDNNECTED BY SPACED CDNDUCTIVE PLATES Michael M. Mann, Hawthorne, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Apr. 3, 1963, Ser. No. 270,309 21 Claims. (Cl. 3153.6)

This invention relates generally to traveling-wave tubes, and more particularly relates to a novel traveling-wave tube slow-wave structure which achieves high power operation while maintaining a high interacton impedance, high efiiciency, and wide bandwidth.

In traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along a slow-wave structure which is disposed along and about the electron stream path. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure, and hence, the travelingwave may be made to effectively propagate at nearly the velocity of the electron stream. Interactions between electrons in the stream and the traveling-wave cause velocity modulation and bunching of the stream electrons, the net result being a transfer of energy from the electron stream to the wave traveling along the slow-wave structure.

A classic form of slow-wave structure is the helix circuit in which a helical conductor is wound coaxially about the path of the electron stream. The helix circuit is noted for its high interaction impedance and its efiicient and wide-band operation. However, traveling-wave tubes with helical slow-wave structures are limited in their power handling capabilities.

In an eifort to provide traveling-wave tubes which are capable of operating at higher power levels, numerous other forms of slow-wave structures have been developed. Some of the most successful include the disk-loaded circuit, the cloverleaf circuit, the ring-bar circuit, and the folded waveguide circuit. Although these circuits have afforded a substantial increase in power level, they do not equal the helix circuit in bandwidth and efficiency characteristics.

A further development which endeavors to combine the advantages of the helix with those of the higher power circuits is the strapped bifilar helix structure in which a plurality of parallel conductive bars diametrically interconnect the two helices at spacings equal to half the pitch of the helices. In addition to affording higher power operation than the conventional monofilar helix, the strapped bifilar helix achieves superior backward-wave mode suppression.

However, in the continuing search for improved slowwave structures, it would be desirable to retain the advantages of the strapped bifilar helix circuit, while at the same time providing a circuit with still greater power handling capabilities, higher interaction impedance, greater efiiciency, superior spurious mode suppression, and in which lower magnetic focusing fields are required. Accordingly, it is a principal object of the present invention to provide such a slow-wave structure.

It is a further object of the present invention to provide a traveling-wave tube slow-Wave structure which, in addition to possessing the advantages set forth above, can be designed to operate with propagating waves having a wide range of phase velocities, thereby allowing interaction with either high or low voltage electron beams.

It is a still further object of the present invention to 3,258,7Gl Patented August 23, 1956 "ice provide a simple and reliable high power traveling-wave tube slow-wave structure which, in addition to possessing the high interaction impedance, high efficiency, and wide bandwidth of conventional low power helix structures, also has a greater design flexibility than such prior art structures to ensure operation with large beam diameters.

It is still another object of the present invention to provide a slow-wave structure for a traveling-wave tube which not only inherently acts to suppress modes giving rise to backward-wave oscillations, but which also lends itself to the accommodation of additional devices for the suppression of undesired modes.

In accordance with the objects outlined above, the traveling-wave tube slow-wave structure provided by the present invention comprises a mul-tifilar helix including -a plurality of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis. A plurality of electrically conductive cross-piece elements are spaced along the mul'tifilar helix in planes penpendicular to the axis of the helix. Each of the crosspiece elements defines an aperture which encompasses the axis, with the apertures in successive crosspieces being aligned with one another. Each crosspiece element further defines a plurality of portions equal in number to the number of individual helical conductors which extend radially fromits aperture to the multifilar helix, with each portion being electrically connected to a different one of the helical conductors. In addition, each radially extending portion in each crosspiece element is angularly displaced with respect to each radially extending portion in the next successive crosspiece element along the axis of the helices.

In preferred embodiments of the invention a bifilar helix is employed, with successive ones of the helix-connecting crosspieces being disposed in quadrature along the bifilar helix at spacings equal to an odd multiple of one quarter of the pitch of the helices. In further embodiments the crosspiece elements are provided with loss means for increasing the electrical resistance to the flow of current through the crosspieces between the helices in order to enhance the suppression of undesired modes.

Other and further objects, advantages, and characteristic features of the present invention will become readily apparent from the following detailed description of preferred embodiments of the invention when taken in conjunction with the appended drawings in which:

FIG. 1 is a longitudinal sectional view illustrating a traveling-wave tube having a slow-wave structure provided according to the principles of the present invention;

FIG. 2 is an enlarged side view of the slow-wave structure of FIG. 1 removed from the tube and partly broken away;

FIG. .3 is a cross-sectional view taken along line 33 of FIG. 2;

FIG. 4 is a cross-sectional view taken along line 44 of FIG. 2;

FIG. 5 is a side view of a slow-wave structure provided by a further embodiment of the present invention;

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 5;

FIG. 7 is a cross-sectional view taken along line 7-7 of FIG. 5;

FIG. 8 is a plan view of a crosspiece for a slow-wave structure provided in accordance with a still further embodiment of the present invention;

FIG. 9 is a sectional view taken along line 99 of FIG. 8;

FIG. 10 is a sectional view similar to FIG. 9 illustrating another embodiment of the present invention; and

FIG. 11 is a sectional view similar to FIG. 9 illustrating still another embodiment of the present invention.

Referring with more particularity to the drawings, in FIG. 1 there is illustrated a traveling-wave tube 10 having a slow-wave structure 11 constructed according to the principles of the present invention. The slow-wave structure 11 is mounted in a tubular metallic envelope 12, which may be of molybdenum, for example, and which may be evacuated to a desired pressure. The slow-wave structure 11 will be described in more detail later on; however, for the present it suffices to state that the slow-wave structure serves to propagate electromagnetic wave energy along and about an electron beam traveling along the longitudinal axis of the envelope 12.

For generating the electron beam a conventional electron gun 14 is disposed in an enlarged portion of the envelope 12 at one end thereof. The electron gun 14 may include an electron-emitting cathode 16, a filamentary heater 18, a focusing electrode 20 and an accelerating anode 22. A voltage source 24 is tapped at appropriate potentials, as shown, to provide operating voltages for the electrodes 16, 20 and 22.

Disposed at the opposite end of the envelope 12 is a collector electrode 24 for intercepting the electrons and dissipating their kinetic energy. A source of potential 26 is connected to the collector 24 to apply a positive bias to the collector 24 to prevent secondarily emitted electrons from the slow-wave structure 11 from returning to the interaction region, thereby reducing noise and other interference.

Input electromagnetic wave energy to be amplified is applied to one end of the slow-wave structure 11 by means of a coaxial input waveguide 28 having an inner conductor 30 which is connected to the slow-wave structure 11. The conductor 30 may be provided with a step transition 31 in its cross-section and may be tapered in a desired manner, as shown at 32, to provide an impedance transformation. A ceramic window 34 may be provided in the waveguide 28 to support a pressure differential and thereby enable the desired evacuated pressure to be maintained within the envelope 12, while allowing electromagnetic wave energy to enter the envelope 12 through the waveguide 28. Similarly, a coaxial output waveguide 28' is provided at the opposite end of the slow-wave structure 11 for propagating amplified electromagnetic wave energy from the slow-wave structure 11 to external circuitry (not shown). The output waveguide 28' is provided with an inner conductor 30' which is connected to the slow-wave structure 11 and which defines a step 31' and tapered portion 32', with a ceramic window 34' also being disposed in the waveguide 28'. It is pointed out that although the input waveguide 28 is shown as disposed at the electron gun end of the traveling-wave tube 10, it would be located at the collector end in the event a backward wave amplifier is desired.

For focusing the emitted electrons and constraining them to flow along an axial path toward the collector 24 there may be provided a periodic permanent magnet focusing arrangement. The focusing assembly includes a plurality of annular disk-shaped permanent magnets 40 respectively interposed between a plurality of annular pole pieces 42 of ferromagnetic material. Each pole piece 42 extends radially inwardly of the magnets 40 and defines a ferrule 44 at its inner extremity. The ferrules 44 protrude outwardly from the planes of the pole pieces 42 along directions parallel to the longitudinal axis of the traveling-wave tube 10, with the inner lateral surfaces of the ferrules 44 abutting against the enevelope 12. It is pointed out that although a periodic permanent magnet focusing arrangement is shown, other focusing schemes, for example non-periodic permanent magnet focusing, solenoid focusing, r electrostatic focusing, may be employed instead.

Referring now to FIGS. 2-4, as well as to FIG. 1, the slow-wave structure 11 for the traveling-wave tube will be described in more detail. The slow-wave structure 11 includes a multifilar helix, illustrated as a bifilar helix consisting of a first helical conductor 50 and a second helical conductor 52. The helices 50 and 52, which may be of a metal such as molybdenum or copper, are co-directionally wound, are equally spaced from each other, and are disposed about a common axis coincident with the electron beam path. The helices 50 and 52 are of the same size, and have the same helical diameter D (the diameter of a circle defined by the projection of points along the inner extremities of the helical conductor in a plane perpendicular to the electron beam axis) and the same pitch P (the axial distance in which the helical conductor makes one complete revolution about its axis).

The helices 50 and 52 are electrically connected together at spaced points along their lengths by a series of electrically conductive crosspiece elements, or webs, 54 (with a prime or a double prime designation being added to indicate the orientation of the webs as will be explained below). The crosspieces 54 may take the form of metal plates, for example of molybdenum or copper. Either the crosspieces 54 or the helices 50 and 52, or both, may be gold-plated to facilitate attachment of the crosspieces 54 to the helices 50 and 52, for example by brazing or spot-welding. The webs 54 are disposed in planes perpendicular to the axis of the slow-wave structure 11 and in their central regions define aligned circular apertures 56 of a diameter d to provide a passage for the axially traveling electron stream. Each web 54 defines first and second portions 54a and 5417, respectively, which extend radially outwardly from opposite sides of its aperture 56 to the bifilar helix. Thus, each web 54 is of greater. extent in a direction through its extended portions 54a and 54b than in an orthogonal direction in the plane in which it is mounted, with its greater, or longitudinal, dimension being essentially equal to the helix diameter D and its smaller, or transverse, dimension being slightly greater than the diameter d of its aperture 56.

A key feature of the present invention, and one which enables many of its significant advantages to be achieved, lies in the orientation of the crosspieces 54 which interconnect the helices 50 and 52. Specifically, the crosspieces 54 are staggered along the length of the slowwave structure in accordance with a predetermined relation, i.e., the longitudinal dimension of each crosspiece is angularly displaced by a predetermined amount with respect to the longitudinal dimension of the next successive crosspiece along the axis of the slow-wave structure. Preferably, the staggering relationship is such that successive crosspieces 54 are disposed in quadrature along the axis of the helices, i.e., the longitudinal dimension of each crosspiece 54 is rotated by with respect to that of the preceding and following crosspieces. Thus, there is provided a first series of crosspieces 54 longitudinally aligned in a first direction perpendicular to the electron beam axis and a second series of crosspieces 54" longitudinally aligned in another direction perpendicular to both the first direction and the electron beam axis, with individual crosspieces in each series being alternately disposed with despect to individual crosspieces in the other series along the length of the slow-wave structure.

The spacing S between individual crosspieces of the slow-wave structure of the present invention may be determined in accordance with the relation:

where P is the helix pitch as defined above and n is an odd integer. In the embodiment of FIGS. l-4, n is equal to one, and therefore, the web spacing is A of the pitch of the helical conductors, with the webs interconnecting the helices every 90 along their lengths.

In prior art helix slow-wave circuits, the pitch of the helix is designed so that the phase velocity v of the wave traveling along the helix is approximately synchronous with the electron beam, in practice being slightly less than the average beam velocity v This is accomplished by making the ratio of helix pitch P to the actual helix length L for one complete revolution approximateliy equal to the ratio of the desired phase velocity v to the velocity of light c.

In the slow-wave circuit of the present invention, however, the helical conductors are not designed to be synchronous with the electron beam; but rather a ratio of a modified waveguiding length L to web spacing S is employed in the design. The modified length L is measured from the center of the beam aperture 56 in a web 54 outwardly to one of the helical conductors 50 or 52, then along the helix St or 52 to the next web 54", and inwardly along the web 54 to the center of its beam aperture 56. The result is that the pitch of the helices of the slow-wave circuit of the present invention is greater than that of a slow-wave circuit designed so that the phase velocity of the wave traveling along the helix is synchronous with the electron beam.

The slow-wave structure 11 may be mounted within the tubular envelope 12 on a plurality of longitudinally extending support rods 60 of insulating material which may be, for example of sapphire or berylli'a. Although four support rods equally circumferentially spaced about the helices 50 and 52 at points opposite their points of attachment to the webs 54 are used in the embodiment illustrated in FIG. 1, other numbers and orientations of helix support rods may alternately be employed.

The inner conductor 30 of the coaxial input waveguide 28 is connected to the web 54" nearest the electron gun 14 at 62, for example by brazing or spot-welding; while the inner conductor 3% of the coaxial output waveguide 28' is similarly fastened at 62 to the web 54" nearest the collector 24. It is pointed out that the points of attachment 62 and 62 of the conductors 30 and 30 need not be located circumferentially along the edge of the web 54" midway between the points of attachment to the helices 50 and 52, as shown, but may be circumferentially disposed at an angle of around 45 With respect to point of attachment of the web 54" to the helix which bends away more rapidly from the web edge in question.

In a further embodiment of the present invention, illustrated in FIGS. 7, the value of n in Equation 1 is, made equal to three so that the web spacing S becomes /4 of the helix pitch P. Thus, in this embodiment helical conductors 7t) and 72 are interconnected at spaced points 270 apart along their lengths by alternately disposed webs 74' and 74" having their longitudinal dimensions rotated by 90 with respect to one another along the axis of the helices. The webs 74 and 74 define aligned circular apertures 76 of a diameter d to accommodate the electron beam.

In the operation of a traveling-wave tube employing a slow-wave structure according to the present invention, electromagnetic Wave energy to be amplified is fed to the input end of the slow-wave structure 11 via coaxial input line 28. This Wave energy then propagates along the slow-wave structure 11 toward the collector electrode 24, and in so doing interacts with electrons in the axially traveling electron beam which traverses the apertures 56. Energy is transferred from the electrons to the traveling electromagnetic waves, thereby amplifying the traveling waves. The amplified electromagnetic wave energy is removed from the traveling-wave tube via coaxial output line 28'.

In order to gain a better understanding and appreciation for the vastly improved results afforded by the slowwave structure of the present invention, a brief and simplified discussion of the theory underlying the present invention will be presented. A bifilar helix will support the propagation of electromagnetic wave energy in both a symmetric mode (having fields propagating in phase with each other along the two helices) and an asymmetric mode (having fields propagating 180 out of phase from each other along the two helices). The fundamental space harmonic of the symmetric mode is the one normally used for forward-wave amplification, while the asymmetric mode is capable of producing backward-Wave oscillations. Thus, in a forward-wave amplifier it is desired to maximize interaction with the fundamental component of the symmetric mode, while minimizing interaction with the asymmetric mode in order to reduce oscillations and other spurious outputs. As is the case with the crossbars of the strapped bifilar helix, the helixconnecting webs 54 of the slow-wave circuit of the present invention tend to provide a short circuit between opposite points on the helices 50 and 52. However, on account of their configuration, the webs 54 of the slow-wave circuit of the present invention have a much lower inductance than do the narrow bars of the strapped bifilar helix, thereby providing a path of lower impedance between the helices which more closely approaches an ideal short circuit. Since in the symmetric mode the same voltage exists on both helices at a given cross-section, while for the asymmetric mode opposite voltages are present, the short circuits will strongly perturb the asymmetric mode While having little effect on the propagation of the asymmetric mode. Thus, the slow-wave circuit of the present invention inherently functions to suppress the asymmetric mode and the undesirable backward-wave oscillations which could otherwise result from its propagation.

In a conventional helix slow-wave circuit when the helix is designed to propagate waves of greater phase velocities, interaction in the fundamental forward space harmonic decreases While interaction in the backward space harmonic increases. However, on account of its modeselective properties, this effect is minimized with the slowwave circuit of the present invention so that the helices may be designed to propagate waves With greater phase velocities. This makes possible interaction with faster traveling (higher voltage) electron beams, resulting in higher power operation than is possible with conventional helix circuits.

In addition, with prior art helical slow-wave structures including the strapped bifilar helix, the fundamental forward space harmonic of the wave propagating along the helix consists of both TE portions (transverse-electric waves in which there is no component of electric field along the direction of propagation) and TM portions (transverse-magnetic Waves in which there is no component of magnetic field along the direction of propagation) in approximately equal amounts. Since the axial magnetic fields do not interact with the electron beam, only the TM Waves (with their axial electric field components) are useful for interacting With the beam. Thus, a substantial part of the fundamental component of the circuit wave energy is Wasted in these prior art slow-wave circuits. However, the slow-Wave circuit of the present invention strongly favors the propagation of fundamental TM waves over fundamental TE waves, and hence almost all of the fundamental space harmonic energy propagating along the slow-wave circuit of the present in-' vention is useful for interaction with the electron beam. Thus, there is provided greater interaction for a given amount of input wave energy, greater efficiency, and a higher interaction impedance than With prior art helical slow-wave structures.

Moreover, since the fundamental space harmonic propagated by the slow-wave structure of the present invention contains a greater portion of TM energy than do fundamental space harmonics of prior art helical sloW- wave circuits, the other space harmonics propagated by the slow-wave circuit of the present invention contain a smaller portion of TM energy than do those of prior art helices. Therefore, there is less interaction with these other space harmonics with the slow-wave circuit of the present invention than with the prior art circuits, thereby further reducing spurious mode interaction.

In further embodiments of the present invention additional means are provided in order to afford an even greater reduction in spurious mode interaction. Thus, in the embodiment illustrated in FIGS. 8 and 9, a web 84 used to interconnect the two helical conductors may be constructed with first and second metal members 84a and 84b, respectively, which extend radially outwardly from opposite sides of its aperture 86 to the respective helical conductors, and with a pair of lossy elements 88 and 89 disposed on opposite sides of the aperture 86 between the met-a1 members 84a and 84b. The lossy elements 88 and 89 each extend outwardly from the aperture 86 in a radial direction orthogonal to the radial direction in which the members 84a and 84b extend, with the radial extent of the lossy elements 88 and 39 being less than that of the metal members 84a and 84b. The elements 88 and 89 may be composed of a material such as kanthal, aquadag or a lossy ceramic such as a mixture of forsterite and silicon carbide.

In the symmetric mode there will be a minimum current flow through the lossy elements 88 and 89 of the connecting webs 84, While in the asymmetric mode substantially greater current will flow through the connecting webs 84. On account of the lossy elements 38 and 89, greater power dissipation will occur for the asymmetric mode than for the symmetric mode. Thus, modeselective loss is introduced into the webs 84 which attenuates the asymmetric mode while having essentially no eifect on the symmetric mode.

In an alternate mode-selective loss arrangement illustrated in FIG. 10, a helix-connecting web 94 is constructed in a single piece, as are the Webs 54 and 74, and the loss is provided by coatings 98 on both broad surfaces of the web 94 in regions extending radially outwardly from opposite sides of its aperture 96 along the transverse, or smaller, dimension of the web 94 in the plane in which it is mounted. The coatings 98 may be of a material such as kanthal and may have a thickness of approximately one skin depth.

In the mode-selective loss arrangement shown in FIG. 11, lossy material 108 is disposed in indentations 107 in the helix-connecting web 104. Each broad surface of the web 104 defines a pair of such indentations 107 which extend radially outwardly from opposite sides of its aperture 106 along a direction orthogonal to the longitudinal, or greater, dimension of the web 104 in the plane in which it is mounted. The loss 108 may be applied, for example, by flame-spraying the entire broad surfaces of the web 104 with kanthal, and subsequently grinding the web 104 to remove the lossy coatings in all regions other than in the indentations 107.

A further advantage of the slow-wave circuit of the present invention over prior art helical slow-wave circuits such as the strapped bifilar helix lies in the fact that an additional design parameter is afforded; namely, the diameter d of the beam apertures 56 may be varied in addition to the diameter D of the helices 50 and 52. For example, the beam hole diameter d may be increased without changing the helix diameter D, thus affording operation with beams of a larger diameter. Not only does a greater beam diameter allow higher power operation to be achieved, but it also enables the use of beams with lower space charge densities so that lower magnetic focusing fields may be employed, such as those readily achievable with periodic permanent mag net focusing.

A further characteristic of the slow-wave structure of the present invention, and one which readily lends itself to high power operation, is its excellent thermal capabilities, resulting from two considerations. First, on account of their relatively large cross-sections, the webs 54 provide a good thermal path away from the interaction region. Second, the field intensity outside of the helices 50 and 52 is such that the slow-wave strucrture housing 12 may be located closer to the helices 50 and 52 than in prior art helical circuits without introducing appreciable loading. This enables smaller support rods 60 to be employed than for the prior art circuits, thereby affording a lower thermal resistance.

The angularly staggered relationship between the successive crosspieces 54' and 54 reduces the opposing area of their broad surfaces. This reduces the capacitance between crosspieces, thereby lowering the stored energy per unit volume along the slow-wave structure. Since the interaction impedance is inversely proportional to stored energy, an increase in interaction impedance results.

As has been mentioned above, for the slow-wave structure of the present invention current paths are provided outwardly along a web from the beam hole to the helices, along the helices to the next web, and inwardly along that web to the beam hole. Thus, the helices and webs function together as the slow-waveguiding circuit, and the circuit may be visualized as a folded helix since the webs cause the current paths to be folded in toward the beam at spaced points along the helices.

As has also been mentioned above, the slow-wave circuit of the present invention enables lower magnetic fields to be employed to focus the electrons in the beam, and thus, traveling wave tubes incorporating such a slowwave structure are ideally suited for periodic permanent magnet focusing. It is further pointed out that the magnetic field strength required for focusing is sufficiently low so that the web 54 need not be of a ferromagnetic material but may be of any metal. Thus, the period of the magnetic focusing circuit comprising the magnets 40 and pole pieces 42 need not bear any particular relationship to the slow-wave circuit period, i.e., the distance between the webs 54. On the other hand, if high magnetic focusing field strengths are desired, ferromagnetic material would be used for the webs 54, in which case the period of the magnetic focusing structure must be equal to or an integral multiple of the period of the slow-wave structure.

The slow-wave circuit of the present invention may also be visualized as a helical meander line in which (instead of interconnecting a plurality of aligned apertured cross-pieces by a pair of conductors which are twisted in a single plane as in a conventional meander line) the conductors are made helical, with the cylinder defined by the helices completely enclosing the crosspieces, and with the crosspices being angularly staggered along the lengths of the helices. Not only does the slow-wave structure of the present invention provide substantially less spurious mode interaction than do conventional meander lines, but in addition, its meander length is greater than that of a conventional meander line. Thus, for the same crosspiece spacing, the phase velocity of waves propagating along the slow-Wave circuit of the present invention is lower than that of waves traveling along a conventional meander line, making possible interaction with beams of a lower voltage (slower average velocity v In this connection it may be further observed that for equal crosspiece spacings S, the modified length L of the wave propagating structure of the slow-wave circuit of FIGS. 5-7 is substantially greater than that of the circuit of FIGS. 2-4. Thus, the embodiment of FIGS. 5-7 is able to operate with beams of lower voltage and is less dispersive (i.e., the frequencyphase characteristic has a smaller second derivative) than the circuit of FIGS. 2-4.

Thus, it will be apparent that on account of its ability to handle large diameter electron beams and its excellent thermal capabilities, the slow-wave structure of the present invention can operate at power levels approaching those of such high power circuits as the cloverleaf, ringbar and folded waveguide circuits. At the same time, the slow-Wave circuit of the present invention provides a high interaction impedance and wide bandwidth comparable to that achievable with helix circuits, while surpassing prior art helix circuits in ability to suppress spurious modes. In addition, the circuit of the present invention is able to operate with low magnetic focusing fields and a wider range of beam voltages than prior art slow-Wave structures.

It will be appreciated that while the embodiments of slow-wave structures illustrated herein have employed bifilar helices, the principles of the present invention are also applicable to multifilar helices employing more than two individual helical conductors. In the general case of an N-filar helix, where N is an integer not less than two, each helix-connecting crosspiece may define N portions equally spaced circumferentially about its electron beam aperture. Each portion would extend radially outwardly from the aperture to the N-filar helix and would be electrically connected to a difierent one of the N individual helical conductors. In addition, each radially extending portion in each crosspiece would be angularly displaced with respect to each radially extending portion in the next successive crosspiece along the slow-wave structure axis, for example by displacing each radially extending portion of a crosspiece by an angle of 180/N with respect to one of the radially extending portions of the next crosspiece.

It will also be apparent that when bifilar helices are used the angle of displacement of successive crosspieces need not be 90 or 270, but rather other staggering angles, for example 45, 60", 120, 225, etc., may be employed.

Thus, although the present invention has been shown and described with reference to particular embodiments, nevertheless, various changes and modifications obvious to one skilled in the art are deemed to be Within the spirit, scope and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. A slow-wave structure for a traveling-wave tube comprising: a multifilar helix including a plurality of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a com mon axis, a plurality of electrically conductive elements spaced along said multifilar helix in planes perpendicular to said axis, each of said elements defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, each of said elements having a plurality of portions equal in number to the number of individual helical conductors extending radially from its aperture to the multifilar helix, with each of said portions being electrically connected to a different one of said helical conductors, and each said radially extending portion in each of said elements being angularly displaced by an angle other than 180 with respect to each said radially extending portion in the next successive one of said elements along said axis.

2. A slow-wave structure for a traveling-wave tube comprising: an N-filar helix Where N is an integer not less than two, said N-filar helix including N equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive webs equally spaced along said N-filar helix in planes perpendicular to said axis, each of said webs defining an essentially circular aperture encompassing said axis, the apertures in successive webs being aligned with one another, each of said webs defining N portions equally spaced circurnferentially about said aperture and extending radially from said aperture to said N-filar helix, with each of said portions being electrically connected to a different one of said helical conductors, and each said radially extending portion of each of said webs being angularly displaced by an angle of l80/N with respect to one of said radially extending portions of the next successive web along said axis.

3. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive elements mounted along said bifilar helix in planes perpendicular to said axis, each of said elements defining an aperture encompassing said axis and aligned with respective apertures in the other ones of said elements, each of said elements further defining first and second portions extending radially from opposite sides of its aperture to said bifilar helix, with said first and second portions respectively being electrically connected to said first and second helical conductors, and said first portion of each of said elements being angularly displaced with respect to both said first and said second portions of the next successive one of said elements along said axis.

4.'A slow-wave structure for a traveling-Wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive crosspieces equally spaced along said bifilar helix in planes perpendicular to said axis, each of said crosspieces being electrically connected to both said first and said second helical conductors, each of said crosspieces defining an aperture encompassing said axis, the apertures in successive crosspieces being aligned with one another, each of said crosspieces having a dimension essentially equal to said helical diameter along at least one direction in the plane in which it is mounted, and said one direction in each crosspiece plane being angularly displaced by an angle other than with respect to the said one direction in the plane of the next successive crosspiece along said axis.

5. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including a pair of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of equally spaced electrically conductive plates mounted along said bifilar helix in planes perpendicular to said axis, each of said plates being electrically connected to both of said pair of helical conductors, each of said plates defining an aperture encompassing said axis,

the apertures in successive plates being aligned with one' another, each of said plates having a dimension essentially equal to said helical diameter along a first direction in the plane in which it is mounted and having a smaller dimension along a second direction in said plane orthogonal to said first direction, said first direction in each of said planes being aligned with said second direction in the plane of the next successive plate along said axis.

6. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including a pair of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a first and a second series of identical electrically conductive elements mounted along said axis in planes perpendicular thereto, each of said elements being electrically connected to both of said pair of helical conductors, each of said elements defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, each of said elements having a greater extent in one direction in the plane in which it is mounted than in a transverse direction in said plane, the greater extent direction for each of the elements in each series being aligned with the greater extent direction for each of the other elements in its series and with the transverse direction for each of the elements in the other series, and individual ones of the elements of said first and second series being alternately disposed along said axis.

7. A slow-wave structure for a traveling-Wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of conductive webs electrically interconnecting said first and said second helical conductors at spaced points therealong, said webs being equally spaced along said bifilar helix in planes perpendicular to said axis and having a greater extent in one direction in the plane in which it is mounted than in an orthogonal direction in said plane, each of said webs defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said webs, and successive ones of said webs being disposed in quadrature along said bifilar helix.

8. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive elements spaced along said bifilar helix in planes perpendicular to said axis, each of said elements being electrically connected to both said first and said second helical conductors, each of said elements defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, and the spacing between said elements being equal to nP/ 4 where n is an odd integer and P is the pitch of said helical conductors.

9. A slow-wave structure according to claim 8 wherein n is equal to one.

10. A slow-wave structure according to claim 8 wherein n is equal to three.

11. A slow-wave structure according to claim 7 wherein said webs are provided with loss means for increasing the electrical resistance to the flow of current through said webs between said first and said second helical conductors.

12. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive means mounted along said bifilar helix in planes perpendicular to said axis, each of said conductive means defining an aperture encompassing said axis and aligned with respective apertures in the other ones of said conductive means, each of said conductive means defining first and second portions of essentially no electrical resistance extending radially outwardly from opposite sides of its aperture to said bifilar helix and being respectively connected to said first and second helical conductors, each of said conductive means further defining third and fourth portions of relatively high electrical resistance disposed between said first and second portions and each extending radially outwardly from opposite sides of said aperture in a radial direction essentially orthogonal to the radial directions in which said first and second portions extend, the radial extent of said third and fourth portions being less than that of said first and second portions, and the radial extent direction of said first portion of each of said conductive means being aligned with the radial extent direction of said third portion of the next successive conductive means along said axis.

13. A slow-wave structure according to claim 12 wherein said first and said second portions of said conductive means are of metal and said third and fourth portions are of lossy ceramic material.

14. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of equally spaced electrically conductive plates mounted along said bifilar helix in planes perpendicular to said axis, each of said plates being electrically connected to both said first and said second helical conductors, each of said plates defining an aperture encompassing said axis, the apertures in successive plates being aligned with one another, each of said plates having a dimension essentially equal to said helical diameter along a first disection in the plane in which it is mounted and having a smaller dimension along a second direction in said plane orthogonal to said first direction, said first direction in each of said planes being aligned with said second direction in the plane of the next successive plate along said axis, and a coating of lossy material having a thickness of approximately one skin depth on both broad surfaces of said plates in regions extending radially outwardly from opposite sides of said aperture along a direction essentially orthogonal to said first direction. 7

15. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of equally spaced electrically conductive plates mounted along said bifilar helix in planes perpendicular to said axis, each of said plates being electrically connected to both said first and said second helical conductors, each of said plates defining an aperture encompassing said axis, the apertures in successive plates being aligned with one another, each of said plates having a dimension essentially equal to said helical diameter along a first direction in the plane in which it is mounted and having a smaller dimension along a second direction in said plane orthoganal to said first direction, said first direction in each of said planes being aligned with said second direction in the plane of the next successive plate along said axis, each said plate defining first and second indentations in each broad surface of said plate, with said first and second indentations extending radially outwardly from opposite sides of the aperture in said plate along a direction essentially orthogonal to said first direction, and lossy material disposed in each of said indentations.

16. A traveling-wave tube comprising in combination: electron gun means for launching a stream of electrons in a predetermined direction; collector means spaced from said electron gun means for intercepting electrons of said stream; focusing means for constraining said stream of electrons to flow along a predetermined path toward said collector means; and a slow-wave structure disposed between said electron gun means and said collector means; said slow-wave structure comprising a multifilar helix including a plurality of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis coincident with said electron stream path, a plurality of electrically conductive elements spaced along said multifilar helix in planes perpendicular to said axis, each of said elements defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, each of said elements having a plurality of portions equal in number to the number of individual helical conductors extending radially from its aperture to the multifilar helix, with each of said portions being electrically connected to a different one of said helical conductors, and each said radially extending portion in each of said elements being angularly displaced by an angle other than with respect to each said radially extending portion in the next successive one of said elements along said axis.

17. A traveling-wave tube comprising in combination: electron gun means for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting electrons of said stream, focusing means for constraining said stream of electrons to flow along a predetermined axis toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream axis between said electron gun means and said collector means, a plurality of insulating support rods mounted within said envelope and disposed parallel to said electron stream axis, a multifilar helix mounted on said insulating support rods, said multifilar helix including a plurality of equally spaced individual helical conductors having the same helical diameter and pitch coaxially disposed about said electron stream axis, a plurality of electrically conductive elements equally spaced along said multifilar helix in planes perpendicular to said axis, each of said elements defining an essentially circular aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, each of said elements having a plurality of portions equal in number to the number of individual helical conductors extending radially from its aperture to the multifilar helix, with each of said portions being electrically connected to a different one of said helical conductors, and each said radially extending portion in each of said elements being angularly displaced by an angle other than 180 with respect to each said radially extending portion in the next successive one of said elements along said axis.

18. A traveling-wave tube comprising in combination: electron gun means for launching a stream of electrons in a predetermined direction; collector means spaced from said electron gun means for intercepting electrons of said stream; focusing means for constraining said stream of electrons to flow along a predetermined path toward said collector means; a slow-wave structure disposed between said electron gun means and said collector means; said slow-wave structure comprising a multifilar helix including a plurality of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis coincident with said electron stream path, a plurality of electrically conductive elements spaced along said multifilar helix in planes perpendicular to said axis, each of said elements defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, each of said elements having a plurality of portions equal in number to the number of individual helical conductors extending radially from its aperture to the multifilar helix, with each of said portions being electrically connected to a different one of said helical conductors, and each said radially extending portion in each of said elements being angularly displaced by an angle other than 180 with respect to each said radially extending portion in the next successive one of said elements along said axis; coaxial input means for propagating input electromagnetic wave energy to the end of said slow-wave structure nearest said electron gun means, the inner conductor of said input means being electrically connected to one of said elements at the electron gun end of said slow-wave structure at a point spaced from said helical conductors; and coaxial output means for propagating amplified electro-rnagnetic wave energy away from the end of said slowwave structure nearest said collector means, the inner conductor of said output means being electrically connected to one of said elements at the collector end of said slow-wave structure at a point spaced from said helical conductors.

19. A slow-wave structure for a traveling-wave tube comprising: a multifilar helix including a plurality of equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive elements spaced along said multifilar helix in planes perpendicular to said axis, each of said elements defining an aperture encompassing said axis and aligned with the respective apertures in the other ones of said elements, each of said elements being electrically connected to each of said helical conductors at at least a point, and each of said points for each of said elements being angularly displaced by an angle other than 180 with respect to each of said points for the next successive one of said elements along said axis.

20. A slow-wave structure for a traveling-wave tube comprising: an N-filar helix where N is an integer not less than two, said N-filar helix including N equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive webs equally spaced along said N-filar helix in planes perpendicular to said axis, each of said webs defining an essentially circular aperture encompassing said axis, the apertures in successive webs being aligned with one another, each of said webs being electrically connected to each of said helical condoctors at at least a point, and each of said points for each of said webs being angularly displaced by an angle of 180/N with respect to one of said points for the next successive web along said axis.

21. A slow-wave structure for a traveling-wave tube comprising: a bifilar helix including first and second equally spaced individual helical conductors having the same helical diameter and pitch disposed about a common axis, a plurality of electrically conductive elements mounted along said bifilar helix in planes perpendicular to said axis, each of said elements defining an aperture encompassing said axis and aligned with respective apertures in the other ones of said elements, each of said elements being electrically connected to said first helical conductor at at least a first point and to said second helical conductor at at least a second point, each of said first and said second points for each of said elements being angularly displaced by an angle other than 180 with respect to each of said first and said second points for the next successive one of said elements along said axis.

References Cited by the Examiner UNITED STATES PATENTS 2,802,135 8/1957 Dodds 333-31 X 2,859,375 11/1958 Brewer 33331 X 2,947,906 8/ 1960 Litton 315--3.5 2,967,259 1/1961 Lagerstrom et al 315-35 HERMAN KARL SAALBACH, Primary Examiner.

R. D. COHN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,268,761 August 23, 1966 Michael M. Mann It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 60, for "despect" read respect column 5, line 3, for "approximateliy" read approximately column 6, lines 22 and 23, for "asymmetric" read symmetric Signed and sealed this lst day of August 1967.

(SEAL) Attest:

EDWARD M. FLETCHER, JR. Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

19. A SLOW-WAVE STRUCTURE FOR A TRAVELING-WAVE TUBE COMPRISING: A MULTIFILAR HELIX INCLUDING A PLURALITY OF EQUALLY SPACED INDIVIDUAL HELICAL CONDUCTORS HAVING THE SAME HELICAL DIAMETER AND PITCH DISPOSED ABOUT A COMMON AXIS, A PLURALITY OF ELECTRICALLY CONDUCTIVE ELEMENTS SPACED ALONG SAID MULTIFILAR HELIX IN PLANES PERPENDICULAR TO SAID AXIS, EACH OF SAID ELEMENTS DEFINING AN APERTURE ENCOMPASSING SAID AXIS AND ALIGNED WITH THE REPECTIVE APERTURES IN THE OTHER ONES OF SAID ELEMENTS, EACH OF SAID ELEMENTS BEING ELECTRICALLY CONNECTED TO EACH OF SAID HELICAL CONDUCTORS AT AT LEAST A POINT, AND ECH OF SAID POINTS FOR EACH OF SAID ELEMENTS BEING ANGULARLY DISPLACED BY AN ANGLE OTHER THAN 180* WITH RESPECT OF SAID POINTS FOR THE NEXT SUCCESSIVE ONE OF SAID ELEMENTS ALONG SAID AXIS. 